1. Technical Field
The present invention relates to the field of jet stream polishing, coating removal and cleaning adherent materials from workpieces, and particularly to abrasive jet stream polishing, wherein a suspension of abrasive particles in a viscous medium is pumped under pressure and at high velocity against the surface of a workpiece to effect polishing and cleaning operations. Such operations are usefully employed in polishing the surfaces of metal sheet and plate, cast and forged metal articles, and related surfaces in the fabrication of useful articles. Typical applications are the polishing surfaces of materials which are difficult to machine, such as stainless steels, titanium, nickel alloys, tungsten carbide, tool steels and the like. The technique may be employed in operations on non-metal surfaces as well, including reinforced polymer composites, ceramics, glass, rock and the like. The technique is also useful for cleaning and descaling metals, removal of paints and coatings, including ceramic coatings, from surfaces, and the like.
2. Prior Art
Polishing and related operations to smooth and finish surfaces are basic to the fabrication of metals into useful articles and have been practiced as long as the production and use of metals has been known. Indeed, in the fabrication of many metal articles, surface finishing operations represent a very substantial component of the total manufacturing operations required. Such operations, in a variety of forms and techniques, are typically labor intensive and expensive.
All such techniques, of which there are large numbers, are based on the removal of elevated portions from the surface, to a level consistent with lower portions. The removal of such elevated portions requires the performance of work on the workpiece surface. Work on lower surface portions is avoided or minimized.
Viewed in such broad terms, the most commonly employed techniques are based on mechanical working of the surfaces, by such common techniques as sanding, grinding, lapping, and the like. Chemical and electrical techniques are also known.
There is need in the art for more productive and economical polishing techniques which can be employed in less labor intensive operations, and to permit faster and more uniform production of finished workpieces with specified surface finish characteristics.
Work is often performed on metals and other materials by techniques involving propelling fluid or fluidized streams onto the surface. Among such techniques are sand blasting and related techniques, and jet stream cutting and machining, including abrasive jet stream cutting and machining.
Sand blasting is typically employed to roughen or peen surfaces or to remove rust and scale from metal surfaces and the like. Such techniques are not generally employed for polishing operations, and are typically directed to operations which produce surfaces which, in fact, do not require polishing in typical operations. Sand blasting is often used, for example, to roughen surfaces to aid in the adherence of applied coatings or adhesive bonding to such surfaces.
Abrasive water jets have grown to be widely employed in cutting and machining operations, particularly with metal sheet and plates to effect rapid and economical cutting and related forming operations. Typical applications have been the cutting of materials which are difficult to machine, such as stainless steels, titanium, nickel alloys, reinforced polymer composites, ceramics, glass, rock and the like. Such techniques are particularly advantageous to produce cutting action through very highly localized action at low average applied force, to effect cutting of such materials with minimal thermal stress or deformation, without the disruption of crystalline structure, and without delamination of composite materials.
To effect abrasive water jet cutting, a specialized nozzle assembly is employed to direct a coherent collimated high pressure stream through a small diameter orifice to form a jet. Typically, pressures of about 200 MPa (about 30,000 psi) and higher are applied to force the media through the nozzle orifice.
Typical nozzle assemblies are formed of abrasion resistant materials, such as tungsten carbide or boride. The orifice itself may be formed of diamond or sapphire.
Abrasive particles are added to the high speed stream of water exiting the nozzle orifice by directing the water stream through a "mixing tube" and introducing abrasive particles into the tube in the region between the exit of the stream from the orifice and its entry into the "mixing tube." The mixing tube, which is typically several inches in length, is a region of contained, extremely turbulent flow in which the relatively stationary or slow moving abrasive particles are accelerated and become entrained in the high speed water stream, which may have nozzle exit velocities as high as Mach 3. The entrainment process tends to disperse and decelerate the water stream while the abrasive particles collide with the tube wall and with each other.
Relatively wide kerfs result from the dispersed stream, energy is wasted, and the tube is rapidly worn, even when made from abrasion resistant materials, such as tungsten carbide or Boride and the like. Some studies have shown that as much as 70% of the abrasive particles are fractured before they reach the workpiece to be cut.
In recent developments, water jets without abrasives have been thickened with water soluble polymers, which aid in obtaining and maintaining coherent jet streams, in reducing the level of misting, splashing and the like. Somewhat narrower kerfs can be achieved. Operating pressures and velocities remain quite high.
It is also known to suspend particulate abrasives in water jets, ordinarily relying on the thickening effect of the water soluble polymers to act as a suspending agent in the system. The abrasive cuts with greater efficiency than the water alone or the water with a thickening agent, but introduces an entire new spectrum of difficulties.